The present invention relates to biomaterials modified with non-proteinaceous catalysts for the dismutation of superoxide, and processes for making such materials. This modification may be by covalent conjugation, copolymerization, or admixture of the non-proteinaceous catalysts with the biomaterial. The resulting modified biomaterials exhibit a marked decrease in inflammatory response and subsequent degradation when placed in contact with vertebrate biological systems.
“Biomaterial” is a term given to a wide variety of materials which are generally considered appropriate for use in biological systems, including metals, polymers, biopolymers, and ceramics. Also included in the term are composites of such materials, such as the polymer-hydroxyapatite composite disclosed in U.S. Pat. No. 5,626,863. Biomaterials are used in a variety of medical and scientific applications where a man-made implement comes into contact with living tissue. Heart valves, stents, replacement joints, screws, pacemaker leads, blood vessel grafts, sutures and other implanted devices constitute one major use of biomaterials. Machines which handle bodily fluids for return to the patient, such as heart/lung and hemodialysis machines, are another significant use for biomaterials.
Common metal alloy biomaterials used for implants include titanium alloys, cobalt-chromium-molybdenum alloys, cobalt-chromium-tungsten-nickel alloys and non-magnetic stainless steels (300 series stainless steel). See U.S. Pat. No. 4,775,426. Titanium alloys are frequently used for implants because they have excellent corrosion resistance. However, they have inferior wear characteristics when compared with either cobalt-chromium-molybdenum alloys or 300 series stainless steel. Cobalt-chromium-molybdenum alloys have about the same tensile strength as the titanium alloys, but are generally less corrosion resistant. They also have the further disadvantage of being difficult to work. In contrast, the 300 series stainless steels were developed to provide high-strength properties while maintaining workability. These steels are, however, even less resistant to corrosion and hence more susceptible to corrosion fatigue. See U.S. Pat. No. 4,718,908. Additional examples of biocompatible metals and alloys include tantalum, gold, platinum, iridium, silver, molybdenum, tungsten, inconel and nitinol. Because certain types of implants (artificial joints, artificial bones or artificial tooth roots) require high strength, metallic biomaterials have conventionally been used. However, as mentioned above, certain alloys corrode within the body and, as a result, dissolved metallic ions can produce adverse effects on the surrounding cells and can result in implant breakage.
In an attempt to solve this problem, ceramic biomaterials such as alumina have been used in high-stress applications such as in artificial knee joints. Ceramic biomaterials have an excellent affinity for bone tissue and generally do not corrode in the body. But when used under the load of walking or the like, they may not remain fixed to the bone. In many cases additional surgery is required to secure the loosened implant. This shortcoming led to the development of bioactive ceramic materials. Bioactive ceramics such as hydroxyapatite and tricalcium phosphate are composed of calcium and phosphate ions (the main constituents of bone) and are readily resorbed by bone tissue to become chemically united with the bone. U.S. Pat. No. 5,397,362. However, bioactive ceramics such as hydroxyapatite and tricalcium phosphate are relatively brittle and can fail under the loads in the human body. This has led in turn to the development of non-calcium phosphate bioactive ceramics with high strength. See U.S. Pat. No. 5,711,763. Additional examples of biocompatible ceramics include zirconia, silica, calcia, magnesia, and titania series materials, as well as the carbide series materials and the nitride series materials.
Polymeric biomaterials are desirable for implants because of their chemical inertness and low friction properties. However, polymers used in orthopedic devices such as hip and knee joints have a tendency for wear and build-up of fine debris, resulting in a painful inflammatory response. Examples of biocompatible polymeric materials include silicone, polyurethane, polyureaurethane, polyethylene teraphthalate, ultra high molecular weight polyethylene, polypropylene, polyester, polyamide, polycarbonate, polyorthoesters, polyesteramides, polysiloxane, polyolefin, polytetrafluoroethylene, polysulfones, polyanhydrides, polyalkylene oxide, polyvinyl halide, polyvinyledene halide, acrylic, methacrylic, polyacrylonitrile, vinyl, polyphosphazene, polyethylene-co-acrylic acid, hydrogels and copolymers. Specific applications include the use of polyethylene in hip and knee joint implants and the use of hydrogels in ocular implants. See U.S. Pat. No. 5,836,313. In addition to relatively inert polymeric materials discussed above, certain medical applications require the use of biodegradable polymers for use as sutures and pins for fracture fixation. These materials serve as a temporary scaffold which is replaced by host tissue as they are degraded. See U.S. Pat. No. 5,766,618. Examples of such biodegradable polymers include polylactic acid, polyglycolic acid, and polyparadioxanone.
In addition to wholly synthetic polymers, polymers which are naturally produced by organisms have been used in several medical applications. Such polymers, including polysaccharides such as chitin, cellulose and hyaluronic acid, and proteins such as fibroin, keratin, and collagen, offer unique physical properties in the biological environment, and are also useful when a biodegradable polymer is required. In order to adapt these polymers for certain uses, many have been chemically modified, such as chitosan and methyl cellulose. These polymers have found niches in a variety of applications. Chitosan is often used to cast semi-permeable films, such as the dialysis membranes in U.S. Pat. No. 5,885,609. Fibroin (silk protein) has been used as a support member in tissue adhesive compositions, U.S. Pat. No. 5,817,303. Also, esters of hyaluronic acid have been used to create bioabsorbable scaffolding for the regrowth of nerve tissue, U.S. Pat. No. 5,879,359.
As is evident from the preceding paragraphs, individual biomaterials have both desirable and undesirable characteristics. Thus, it is common to create medical devices which are composites of various biocompatible materials in order to overcome these deficiencies. Examples of such composite materials include: the implant material comprising glass fiber and polymer material disclosed in U.S. Pat. No. 5,013,323; the polymeric-hydroxyapatite bone composite disclosed in U.S. Pat. No. 5,766,618; the implant comprising a ceramic substrate, a thin layer of glass on the substrate and a layer of calcium phosphate over the glass disclosed in U.S. Pat. No. 5,397,362; and an implant material comprising carbon fibers in a matrix of fused polymeric microparticles. The diverse uses of biomaterials require a range of mechanical and physical properties for particular applications. As medical science advances, many applications will require new and diverse materials which can be safely and effectively used in biological systems.
Biomaterials, especially polymers, have been chemically modified in several ways in order to give them certain biological characteristics. For instance, thrombogenesis has posed a perennial problem for the use of biomaterials in hemodialysis membranes. In order to decrease thrombogenesis, hemodialysis fluid circuit materials have been modified by ionic complexation and interpenetration of heparin, U.S. Pat. No. 5,885,609, and by graft copolymer techniques in which heparin is linked to the backbone polymer by polyethylene oxide, Park, K. D., “Synthesis and Characterization of SPUU-PEO-Heparain Graft Copolymers”, J. Polymer. Sci., Vol. 20, p. 1725-37 (1991). Similarly, polymers containing incorporated drugs for elution into the body have been co-implanted with stents in order to prevent restenosis, U.S. Pat. No. 5,871,535.
Although most biomaterials in current use are considered non-toxic, implanted biomaterial devices are seen as foreign bodies by the immune system, and so elicit a well characterized inflammatory response. See Gristina, A. G. “Implant Failure and the Immuno-Incompetent Fibro-Inflammatory Zone” In “Clinical Orthopaedics and Related Research” (1994), No. 298, pp. 106-118. This response is evidenced by the increased activity of macrophages, granulocytes, and neutrophils, which attempt to remove the foreign object by the secretion of degradative enzymes and free radicals like superoxide ion (O2−) to inactivate or decompose the foreign object. Woven dacron polyester, polyurethane, velcro, polyethylene, and polystyrene were shown to elicit superoxide production from neutrophils by Kaplan, S. S., et al., “Biomaterial-induced alterations of neutrophil superoxide production” In “Jour. Bio. Mat. Res.” (1992), Vol. 26, pp. 1039-1051. To a lesser extent, polysulfone/carbon fiber and polyetherketoneketone/carbon fiber composites were shown to elicit a superoxide response by Moore, R., et al, “A comparison of the inflammatory potential of particulates derived from two composite materials” In “Jour. Bio. Mat. Res.”. (1997), Vol. 34, pp. 137-147. Hydroxyapatite, tricalcium phosphate, and aluminum-calcium-phosphorous oxide bioceramics were shown to be degraded by macrophages by Ross, L., et al, “The Effect of HA, TCP and Alcap Bioceramic Capsules on the Viability of Human Monocyte and Monocyte Derived Macrophages” in “Bio. Sci. Inst.” (1996), Vol. 32, pp. 71-79. Similarly, cobalt-chrome alloy beads were degraded by neutrophils in a study by Shanbhag, A., et al, “Decreased neutrophil respiratory burst on exposure to cobalt-chrome alloy and polystyrene in vitro” In “Jour. Bio. Mat. Res.” (1992), Vol. 26, 2, pp. 185-195. Even biomaterials which have been modified to present biologically acceptable molecules, such as heparin, have been found to elicit an inflammatory response, Borowiec, J. W., et al, “Biomaterial-Dependent Blood Activation During Simulated Extracorporeal Circulation: a Study of Heparin-Coated and Uncoated Circuits”, Thorac. Cardiovasc. Surgeon 45 (1997) 295-301. In addition, chemical modification has posed several difficulties. Because of the unique chemical characteristics of each biomaterial and bioactive molecule, covalent linkage of the desired bioactive molecule to the biomaterial is not always possible. In addition, the activity of many bioactive molecules, especially proteins, is diminished or extinguished when anchored to a solid substrate. Finally, the fact that many biologically active substances are heat liable has prevented their use with biomaterials that are molded or worked at high temperatures.
The impact of continual attempts by the organism to degrade biomaterial implants can lead to increased morbidity and device failure. In the case of polyurethane pacemaker lead wire coatings, this results in polymer degradation and steady loss of function. In the use of synthetic vascular grafts, this results in persistent thrombosis, improper healing, and restenosis. As mentioned above, orthopedic devices such as hip and knee joints have a tendency for wear and build-up of fine debris resulting in a painful inflammatory response. In addition, the surrounding tissue does not properly heal and integrate into the prosthetic device, leading to device loosening and opportunistic bacterial infections. It has been proposed by many researchers that chronic inflammation at the site of implantation leads to the exhaustion of the macrophages and neutrophils, and an inability to fight off infection.
Superoxide anions are normally removed in biological systems by the formation of hydrogen peroxide and oxygen in the following reaction (hereinafter referred to as dismutation):O2−+O2−+2H+→O2+H2O2 This reaction is catalyzed in vivo by the ubiquitous superoxide dismutase enzyme. Several non-proteinaceous catalysts which mimic this superoxide dismutating activity have been discovered. A particularly effective family of non-proteinaceous catalysts for the dismutation of superoxideconsists of the manganese(II), manganese(III), iron(II) or iron(III) complexes of nitrogen-containing fifteen-membered macrocyclic ligands which catalyze the conversion of superoxide into oxygen and hydrogen peroxide, described in U.S. Pat. Nos. 5,874,421 and 5,637,578, all of which are incorporated herein by reference. See also Weiss, R. H., et al, “Manganese(II)-Based Superoxide Dismutase Mimetics: Rational Drug Design of Artificial Enzymes”, (1996) Drugs of the Future 21, 383-389; and Riley, D. P., et al, “Rational Design of Synthetic Enzymes and Their Potential Utility as Human Pharmaceuticals” (1997) in CatTech, I, 41. These mimics of superoxide dismutase have been shown to have a variety of therapeutic effects, including anti-inflammatory activity. See Weiss, R. H., et al, “Therapeutic Aspects of Manganese (II)-Based Superoxide Dismutase Mimics” In “Inorganic Chemistry in Medicine”, (Farrell, N., Ed.), Royal Society of Chemistry, in Press; Weiss, R. H., et al, “Manganese-Based Superoxide Dismutase Mimics: Design, Discovery and Pharmacologic Efficacies” (1995) In “The Oxygen Paradox (Davies, K. J. A., and Ursini, F., Eds.) pp. 641-651, CLEUP University Press, Padova, Italy; Weiss, R. H., et al, “Manganese-Based Superoxide Dismutase Mimetic Inhibit Neutrophil Infiltration In Vitro”, J. Biol. Chem., 271, 26149 (1996); and Hardy, M. M., et al, “Superoxide Dismutase Mimetics Inhibit Neutrophil-Mediated Human Aortic Endothelial Cell Injury In Vitro”, (1994) J. Biol. Chem. 269, 18535-18540. Other non-proteinaceous catalysts which have been shown to have superoxide dismutating activity are the salen-transition metal cation complexes, described in U.S. Pat. No. 5,696,109, and complexes of porphyrins with iron and manganese cations.